Move error handler into separate file

This commit moves the global error handler into a separate file,
so that it can catch module loading errors.

This also adds support for properly displaying error messages with
newlines in them (since the module loader may throw those)

vnc.html

@@ -59,6 +59,9 @@
         src='http://getfirebug.com/releases/lite/1.2/firebug-lite-compressed.js'></script>
     -->
 
+    <!-- this is included as a normal file in order to catch script-loading errors as well -->
+    <script type="text/javascript" src="app/error-handler.js"></script>
+
     <!-- begin scripts -->
     <script src="vendor/browser-es-module-loader/dist/browser-es-module-loader.js"></script>
     <script type="module" src="app/ui.js"></script>